Boundary layer separation is a fundamentally limiting mechanism which constrains the design of fluid flow systems. As an example, it is known in the helicopter art that retreating blade stall establishes limits on rotor load and flight speed. In addition to the loss of capability to generate lift, unsteady blade stall transmits very large impulsive blade pitching moments to the flight control system. In order to prevent excess control loads, stall boundaries are set as a function of rotor load and flight speed. Stall boundaries define the maximum blade loads, which impact maneuverability and agility as well as speed and payload. Improved payload capability can arise from gains in aerodynamic efficiency in hover via reduction of tip stall and in forward flight via reduction in retreating blade stall. Similar boundary layer problems attend other aerodynamic surfaces, such as fuselages, compressor and turbine blades, wings, and so forth.
Fluid flow in the boundary layer adjacent to a surface exhibits a reduction in velocity due to friction of the molecular viscosity interacting with the surface, which results in a strong velocity gradient as a function of perpendicular distance from the wall:
essentially zero at the surface, raising to mainstream velocity at the outer edge of the boundary layer. The reduced velocity results in a lower momentum flux, which is the product of the density of the fluid times the square of its velocity. Along a diverging surface (that is, a surface that tails away from the mean flow direction), as is the case on the suction surface (the upper surface) of a wing or a helicopter rotor blade, the flow along the surface is accompanied by a pressure rise, which is accomplished only by conversion of momentum flux. The momentum and energy of the fluid along the surface is consumed in overcoming the pressure rise and friction so that the fluid particles are finally brought to rest and the flow begins to break away from the wall, resulting in boundary layer separation. Boundary layer separation typically results in the termination of pressure rise (recovery) and hence loss in performance (e.g., airfoil lift) and dramatic decrease in system efficiency, due to conversion of flow energy into turbulence, and eventually into heat. It is known that boundary layer separation can be deterred by increasing the momentum flux of the fluid particles flowing near the surface. In the art, the deterrence or elimination of boundary layer separation is typically referred to as "delaying the onset of boundary layer separation".
The simplest and most common method for overcoming boundary layer separation includes small vortex generators, which may typically be tabs extending outwardly from the surface (such as the upper surface of an airplane wing), which shed an array of streamwise vortices along the surface. The vortices transport the low momentum particles near the surface away from the surface, and transports the higher momentum particles flowing at a distance from the surface toward the surface, thereby improving the momentum flux of particles flowing near the surface in the boundary layer. This has the effect of deterring boundary layer separation at any given velocity and angle of attack. However, as is known, tab-type vortex generators create parasitic drag which limits the degree of boundary layer separation that can be efficiently/practically suppressed.
Another known approach employs continuous flow into or out of the boundary layer. A wall suction upstream of the boundary separation line (that is the line at which the onset of full boundary layer separation occurs across the surface of an airfoil or a diffuser) simply removes low momentum flux fluid particles from the flow adjacent to the surface, the void created thereby being filled by higher momentum flux particles drawn in from the flow further out from the surface. A similar approach is simply blowing high energy fluid tangentially in the downstream direction through a slot to directly energize the flow adjacent to the surface. Both of these flow techniques, however, require a source of vacuum or a source of pressure and internal piping from the source to the orifices at the surface. These techniques introduce cyclic vortical disturbances into the boundary layer which are amplified in the unstable shear layer into large vortical structures that convect momentum toward the surface; the separation is thereby limited to an extent, but the boundary layer is far from attached. This greatly increases the cost, weight and complexity of any such systems which have not as yet been found to be sufficiently effective to justify use.
A relatively recent, so-called "dynamic separation control" uses perturbations oscillating near the surface, just ahead of the separation point, as are illustrated in U.S. Pat. No. 5,209,438. These include: pivotal flaps which oscillate from being flush with the surface to having a downstream edge thereof extending out from the surface; ribbons parallel to the surface, the mean position of which is oscillated between being within the surface and extending outwardly into the flow; perpendicular obstructions that oscillate in and out of the flow; and rotating vanes (microturbines) that provide periodic obstruction to the flow, and oscillatory blowing. These devices introduce a periodic disturbance in vorticity to the flow, the vortices being amplified in the unstable separating shear layer into large, spanwise vortical structures which convect high momentum flow toward the surface, thereby enabling pressure recovery. Such a flow is neither attached nor separated, under traditional definitions. However, such perturbations must be actively controlled as a function of all of the flow and geometric parameters, dynamically, requiring expensive modeling of complex unsteady flow structures and/or significant testing to provide information for adapting to flow changes either through open loop scheduling or in response to feedback from sensors in the flow.
A recent variation on the dynamic separation control is the utilization of a so-called "synthetic jet" (also referred to as "acoustic jet" or "streaming") directed perpendicular to the surface upstream of the boundary separation line of the surface. This approach has been reported as being highly parameter dependent, thus also requiring dynamic control; and, the results achieved to date have not been sufficient to merit the cost and complexity thereof.